I have a Tannoy SFX5.1 Sub Woofer which, by design, only "powers up" when an audio input signal is detected. To enable this operation there's an internal +/- 12 Volt power supply which is permanently in the on state.
A little while ago my sub-woofer, while in stand-by, started to randomly make noises like distant thunder - also, it wouldn't power up. I "googled" the symptoms, and discovered that I wasn't alone in experiencing this problem - the exact fault (in the stand-by power supply) is described in an excellent blog, and some youtube videos.
I followed the instructions in Andy Doswell's Doz' Blog, which enabled me to repair the fault by replacing a couple of transistors (C1815/NPN A1015/PNP) on the power supply board. At the same time I tested the electrolytic capacitors - a few had failed, so I replaced them all.
After the repair all worked OK for about a week, then the fault reoccured. This time it was just the -12volt supply not working - the A1015 (Q152) transistor had blown, so I replaced it with a CV7672 (equivalent to 2N2905A), which has a higher voltage/current specification and a max collector power dissipation of 600mW. Note that it also has a different pin configuration.
UPDATE: Since I performed the repair I see that PeterCB, a contributor to the Doz' Blog, has had success in replacing Q151 with a with a TIP31 and Q152 with a TIP32 - see the blog. These transistors can dissipate 2 Watts (without a heatsink), so should only just get warm in this application.
For the last five months my Tannoy Woofer has worked without a problem - I keep it in stand-by 24/7.
I couldn't find a circuit diagram online, so I drafted one - you can download/print it here. As a bonus, I've included component values and some voltage/current readings that might be useful.
Let me know of any errors, or suggested improvements, via the Contact Us form on this site.
Last Modified: Friday 26th March 2021 at 11:39 UTC